US2831999A - Thyratron structure - Google Patents

Description

April 1958 H. D. DOOLITTLE 2,831,999

THYRATRON STRUCTURE '3 Sheets-Sheet 1 Filed June 22, 1954 ATTORNEY A ril 22, 1958 H. D. DOOLITTLE 2,831,999

THYRATRON STRUCTURE Filed June 22, 1954 3 Sheets$heet 2 INVENTOR. HOWARD D. DOOLITTLE yfiwn ATT nmsv April 22, 1958 H. D. DOOLITTLE 2,831,999

THYRATRON STRUCTURE Filed June 22, 1954 3 Sheets-Sheet 3 FIG. 4

INVENTOR. H OWARD D. DOOLITTLE yKMQ-WW ATTORNEY United States Patent THYRATRDN STRUCTURE Howard D. Doolittle, Stamford, Conn., assignor t0.

Machlett Laboratories, Incorporated, Springdale, (Jenn, a corporation of Connecticut This invention relates to a thyratron structure wherein a firing electrode other than shielding means is employed to fire the tube.

In prior art low pressure, gas-filled thyratrons, firing has normally been accomplished by application of a voltage pulse to the shielding structure. This shielding structure commonly surrounds and is close spaced (less than five time the mean free path of gas ions at the tubes gas pressure) to the anode and its support structure. Until firing time, the shield is commonly kept at or near cathode potential. Then, when it is desired. to fire the tube, the potential of the shielding means is raised by a voltage pulse and discharge can occur between the cathode and shielding means. The discharge is not immediately transferred to the anode because the ionization resulting from the voltage pulse does not create a sufficiently high charge density in the plasma to provide a low conductance between cathode and anode. However, as ionization increases the discharge is finally transferred to the anode because the tube conductance is increased to a high value such that the main anode-cathode arc can form at low voltage drop.

Because of the physical relationship of the shielding means and anode, the anode conduction tends to bring the shielding means up to a high potential. The increase in shield potential increases conduction from the cathode to the shielding means to a full arc discharge. This are build up takes place over a .02 to .15 micro-second period. Considerable energy is dissipated at the shielding means during this period before conduction is completely transferred to the anode. This energy appears as heat and may actually raise the shielding means to a temperature which supports thermionic emission.

One of the most serious problems encountered in the prior art thyratrons has been their tendency to fire erratically and at undesirable times. An analysis of the problem has convinced me that erratic firing occurs because a high field gradient exists between the shielding means and the anode, and/or because of excessive temperatures of said shielding means. Erratic discharges between the anode and the shielding means are produced by this high field gradient and/or thermionic emission. These discharges, in turn, tend to raise the potential of the shielding means, thereby firing the tube erratically and at an undesirable time.

In order to eliminate the erratic discharges between the cathode and shielding means, certain prior art tubes have been tightly bafiied, i. e., cathode potential bafiles have been employed to make the discharge path between the active surface of the cathode and the shielding means long and devious. As a result of this tight bathing, such tubes are much more difficult to fire when it is desired to fire them.

The present invention permits ease in firing of thyratrons without the danger of erratic firing. The structure of the present invention alsoprovides a uniform firing time. The necessity of using excessively high firing ice : energy because of tight cathode and/or shield potential baffling is eliminated by the present invention. In its preferred embodiment, the present invention prevents high transient voltages between the cathode and the anode baffiing.

These advantages areaccomplished by the use of a novel tube element hereafter called a firing electrode. The firing electrode is located between the cathode and the shield or adjacent to the conduction path. If the firing electrode is judiciously located, usually close to the apertures in the shielding means, firing may be accomplished by applying a voltage pulse of conventional size to the firing electrode. When a firing electrode is employed, erratic firing may be avoided by connecting the shielding means to ground, cathode potential or some other low or negative potential by. a low impedance connection. If the shielding means is so connected through a sufficiently low impedance connection, its potential cannot be easily raised by the voltage pulse. Thus, the

shielding means will not participate in the firing and hence will not tend to overheat. kept small, it also will not tend to overheat.

the shield from the anode make interaction between the firing electrode and the anode much less subject to erratic breakdown than were the shield and anode in prior art structures. Moreover, erratic breakdown will be less likely because the shielding means does not actively participate in the firing. Since firing will occur between the cathode and the firing electrode and be transferred from the firing electrode to the anode, the firing electrode will reduce any erratic conditions which occur.

in the prior art, so-callsd four electrode thyratrons have been employed, but in such structures the fourth electrode or grid is so arranged that it is completely shielded from the cathode. The location of the shield structure between the grid and the cathode makes such structures quite difficult to fire. In fact, the arrangement of the electrodes is such that breakdown will not occur to the grid and such that it will preferably merely serve to supply the firing pulse. Accordingly, the fourth electrode in such structures serves a different purpose from the firing electrode of the present invention which has a different location and is intended to accept breakdown in the way are build up takes place between the cathode:

and the shielding electrode before the conduction is transferred to the anode. This time variation in firing has produced inaccuracies in the functioning of equipment. Consequently, uniform firing time in thyratrons has been widely sought even at the expense of longer deionization time so that greater accuracy can be obtained in the results of equipment which depend upon uniform firing time.

The thyratron of the present invention is capable of more uniform firing time than prior art structures, since the firing electrode lies wholly outside the anode shielding structure. The reason for the improvement in uniformity of performance is the use of a firing electrode which is sufliciently shielded from the anode that it cannot have trode behaves uniformly and is passed on. to the anode If the firing electrode is However, the smaller size and usually more distant spacing than essentially in the same time at each firing. Of course, ionization and deionization time may be greater than in conventional tubes because the firing electrode has a smaller area to aid ,in deionization and because the shielding means does not aid in building up ionization. However, as was previously mentioned, a long deionization time is often not a serious problem and ionization time is not critical as long as it is consistent.

In some cases, despite the attendant disadvantages, it may be desirable to apply the firing pulse to the anode shielding structure, e. g., where short ionization and deionization times assume importance. Should it be desirable to apply the firing pulse to the anode shielding structure, a firing electrode may still be used to advantage. In such event it will probably be necessary to employ tighter baffling in order to avoid erratic firing. Accordingly, a small firing electrode may be located to advantage within the cathode structure in or immediately adjacent to the conduction path. Firing may then be readily started between the firing electrode and the active cathode surface. Once it reaches the firing electrode, the conduction can pass relatively easily on to the grid, and thence to the anode.

All sorts of geometries and arrangements of the firing electrodes can be envisioned. For instance, the firing electrode may be a simple probe member located in or adjacent to the conduction path. Where the aperture in the shielding structure is annular in shape, it may be convenient to employ an annular firing electrode. In fact, a part of the shielding structure, such as a baffle, may serve as the firing electrode, but if the firing electrode is part of the shielding structure it is preferably a relatively small part of it, and, of course, it is electrically insulated from the rest of the shielding structure.

For a better understanding of the present invention reference is made to the following drawings:

Fig. 1 illustrates in axial section a thyratron tube structure in which one of the preferred versions of the present invention is employed.

Fig. 2 is a somewhat schematic vertical section illustrating the cathode, anode, shield, firing electrode and related structure employed in a modified form of the present mvention.

Fig. 3 is a sectional schematic representation of the cathode, shield and anode regions of a modified form of the present invention.

Fig. 4 is another schematic representation of the electrode structure in still another modified form of the present mvention.

Referring now to Fig. l, a structure representing a rather typical type of hydrogen thyratron is illustrated. For the sake of simplicity and clarity, many of the structural details commonly employed in this tube have been omitted because they relate in no way to the novelty of the'present invention. The active elements of the thyratron are enclosed Within a gas-tight envelope which is composed of glass which readily seals to metal. The respective ends of the generally cylindrical envelope 10 are closed in reentrant portions 11 and 12.

Reentrant portion 11 is terminated in stem press 13. Stem press 13 is, in turn, penetrated by metallic leads or rods 14, and 16. These rods may be composed of tungsten or other suitable material. Outside of the vacuum envelope they may be attached to flexible leads 14a, 15a, and 16a, respectively. Leads 14 and 15 are connected to the cathode structure, generally designated 17. Lead 14 is specifically connected to the cylindrical side walls of the cathode structure or to the planar bottom member 19 (as shown) which closes one end of the cyhndrical member 18. The internal walls of cylinder 18 may be coated with a suitable oxide emitter which, when heated to the appropriate temperature, will supply the necessary electrons to operate the tube. The cylindrical member 18 at that end opposite the end which is closed by member 19 may be open except for a laterally extending filament support strip 20. The rod-like lead '15 is advantageously made to penetrate end wall 19 through insulating bushing 21 and the filamentary cathode heater 22 is made to extend between lead 15 and filament support strip 20. The end of the filament supported by strip 26 may have a laterally extending tab 22a which may be readily welded or otherwise affixed to the support strip.

The anode 25 and its associated shielding structure generally designated 34 are supported by that end of the gas tight envelope which is opposite the end supporting the cathode structure. Anode 25, which may be composed of any suitable refractory metal, is conveniently mounted on block 26 which is in turn afiixed to heavy wall cylindrical member 27 by a vacuum tight seal. Members 26 and 27 in some instances have been composed of steel. Copper may be employed, however, in order to minimize gaseous diffusion through these members. If steel, or any other metal which will not readily seal to glass, is employed, it is necessary to affix a collar 28 of glass-sealing metal to tubular member 27. The glass-sealing metal 28 is in turn sealed to an S-shaped extension 29 of reentrant portion 12 of the gas-tight envelope. An axial lead 30 is the conductor supplied to connect the anode to its appropriate circuitry.

Shielding structure 34 is advantageously supported by inwardly extending tubular glass member 32, an appendage to reentrant portion 12 of the gas-tight envelope. Sealed to this tubular glass member 32, is one side wall of annular channel-like metallic member 33. The other side wall of the channel member 33 is sealed to tubular metallic member 34 which advantageously extends below the open end of cathode cylinder 18. Just below the anode surface an annular ring 35 is fixed to the internal surface of tubular member 34. Annular ring 35, in turn, supports a circular bafile member 36 which is spaced away from the annular member 35 by spacers 37. Each of the members 33, '34, and 35, which may be composed of nickel, is kept close spaced to anode 25, so that the distance between the shielding structure and the anode is nowhere more than five times the mean free path of gas ions at the tubes gas pressure.

Connected to lead 16 which extends through the stem press 13 is metallic lead 40 which is preferably a rigid rod. An insulating bushing 41 is employed to separate rod 40 from bracket 42. The rod may be conveniently supported from the cathode structure, and specifically from tubular member 18, by metallic bracket member 42. The rod is advantageously made to have a laterally extending portion 40a which extends over the open end of the cathode structure between the cathode and the anode.

A gas generator of any conventional type may be employed with this tube structure, but for the sake of simplicity is not shown.

In the structure as illustrated, shielding structure 34 and the cathode structure 17 may be conveniently electrically connected together througha low impedance connection (not shown) so that they will remain at the same potential. In the alternative, the shielding structure 34 may have a low impedance lead penetrating the vacuum envelope by which it may be supplied any desired potential such as a negative bias, which will serve to aid its function of holding off firing of the tube until the intended time. It is important that the electrical connection be of low impedance in order to maintain the shielding structure at the desired potential since the anode would otherwise tend to pull its voltage upward toward its own, thus defeating much of the advantage of the firing electrode.

In the structure illustrated there will be no tendency for the shielding structure to rise in potential and cause premature firing of the tube. Instead, the tube will be prevented from firing until the proper potential, pref erably in pulse form, is applied to the probe member 40a.

Thereupon breakdown will occur between the cathode and theprobe and thereafter between the cathode and the anode. Because the probe is of small area and also because it is shielded from the anode so that its potential will not be greatly affected by the anode, relatively little energy will be absorbed by the probe.

Location of the probe 49a may be considerably varied. Moreover, it may be possible to modify both its location and its shape. In fact, baffle member 36 might be used as the firing electrode by attaching it to lead 40. If this were done the spacer members 37 would have to be insulators, of course.

Fig. 2 illustrates in section the active elements of the thyratron without showing the envelope and supporting members. In this drawing many of the structural parts are essentially the same as those shown in Fig. 1. Parts corresponding to those in Fig. l are designated with the same numbers with the addition of primes thereafter.

The cathode and heater (not shown) can be constructed as shown in Fig. 1 or in any of the great variety of conventional forms well known in the art. The leads are similar to those of the Fig. 1 construction. Leads 14 may be connected to cathode end wall 19' Whereas lead is insulated therefrom by bushing 21'. In this instance, the tubular cathode member 18' is shown joined to the tubular shield member 34 by a frusto-conical member 23. This connection provides a low impedance electrical path which insures that the anode shielding structure is maintained at cathode potential.

The anode 25' is supported on block 26 which may be connected to the envelope portion as in the Fig. 1 construction. Shank 30 provides an electrical lead to the anode.

The grid potential structure may be supported by annular flange 33' or by support of the cathode, or both.

The region of particular interest in this case is that region which lies between the cathode and the active anode. In this instance, an annular metal member 35' of shield potential is affixed to the tubular member 34. Disc-like baffle 36' may be spaced from annular member 35 by metallic spacer members 46. The firing electrode in this instance is a metallic ring 44 which may be supported upon insulator members 43 to which they are connected by fine wire ties or other suitable means. Lead 40 is brought through wall 34- through an insulating bushing 41'.

Referring to Figs. 3 and 4, two modified forms of the present invention are illustrated. Referring specifically to Fig. 3, an anode structure is enclosed in a shielding structure generally designated 51 with only a rod-like shank 52 serving as its support and electrical connection to circuitry outside of the gas-tight envelope and extending through the shielding means. The shield is composed of tubular member 53, an annular channel shaped member 54, and perforated disc member 55, all of which are close spaced to the anode structure, and a perforated bafile member 56 which is the shape of a pie pan and which is afiixed to bathe member with the respective perforations of the members offset from one another. The cathode structure generally designated 57 consists of a tubular side wall 58 and planar end walls 59 and 66. End wall. 60 has an axially located circular aperture which is bordered by tubular flange 69a. A plurality of filament elements 61 are arranged in a circular coaxial pattern within the cathode. One end of each filament 61 extends through a respective insulation member 62 in end wall 59. The other end may be attached to the cathode structure and preferably to a bafile member 63 of generally frusto-conical shape, as shown, and with an axial aperture. Between the apertures in frusto-conical baffle 63 and tubular flange 6th: is located a planar baffle member 64 which is supported by a plurality of legs 65 connected to some portion of the cathode structure. The filaments are provided with connections penetrating the gas-tight envelope so that the filaments may be energized in a conventional manner. The shielding structure also has a lead 70 which provided that the firing electrode member 71 were locatedv within the cathode structure adjacent the conduction path. Firing electrode 71 is advantageously connected through lead 73 to a voltage divider 74 which is connected between shielding means 51, or its lead 70, and ground.

The structure shown in Fig. 3, if it did not have the firing electrode 71, would be very difiicult to fire because of the tight bafiling employed, i. e., so much bafiiing is employed that the conduction path is made extremely devious and long. However, when the firing electrode 71 is placed relatively closer to the emitter area, which is preferably the internal surface of cylinder 58, than is the grid, then firing becomes much less of a problem. Breakdown will occur between the cathode emitter and the firing electrode '71, moving thence through the bafliing structure to the shielding structure and ultimately to the anode. Thus, in this case, although it may be smaller in magnitude, the firing electrode has a high potential pulse applied to it as does the shielding structure. However, the firing electrode in this case serves an additional, purpose from that of the firing electrode described in connection with the structures illustrated in Figs. 1 and 2, to wit, it permits firing with a lower potential than would otherwise be possible with such tight baffling. In general, however, the purpose of producing firing more easily has been accomplished in this case just as: it is in those previous cases.

The structure of Fig. 4' is of the same general nature as the structure of Fig. 3 and similar members are given a similar designation with the addition of primes. The bathing in this instance is not quite as tight as it was in the case of the Fig. 3 construction, however, so that firing is not as difiicult. Nevertheless it might still be difficult to fire a tube of the construction shown because of the spacings of the parts employed rather than the geometries as such. In this case, however, potential is applied to the firing electrode directly and to the shielding structure through dropping resistors 75. Of course, in many instances impedance combinations other than pure resistances might be employed.

In another version of the tube structure in which the firing electrode extends into the cathode region, said firing electrode may be directly coupled to the shielding structure and even supported on the shielding structure.

Several varieties of the present invention have been shown in order to illustrate that there are a great many shapes and locations for the firing electrode of the present invention. Many other combinations of the structural parts employing a firing electrode will occur to those skilled in the art. All such versions within the scope of the claims are intended to be within the scope and spirit of the present invention.

1' claim:

1. A thyratron structure comprising a gas-tight envelope, an anode and a cathode located in spaced relation within the envelope, the cathode comprising a hollow cylindrical emitter extending longitudinally of the structure, the emitter being open on the end facing the anode and being electron emissive interiorly, a unitary tubular shield extending substantially coaxial with the emitter with one end portion enclosing the anode and having an integral tubular portion at its other end terminating adjacent the open end of the emitter, a baflie structure carried by the shield and embodying aperture means for providing a circuitous path for the discharge passing from the emitter to the anode, and a firing electrode located within the shield between the bafiie structure and the emitter in electrically isolated relation to the other electrodes and the shield and having an effective portion lying in the electron path adjacent said aperture means in 7 the baffle structure at a point where ionization created by the firing electrode will be transferred in a relatively short time to the anode.

2. A thyratron structure comprising a gas-tight envelope, an anode and a cathode located in spaced relation within the envelope, the cathode comprising a hollow cylindrical emitter extending longitudinally of the structure, the emitter being open on the end facing the anode and being electron emissive interiorly, a unitary tubular shield extending substantially coaxial with the emitter with one end portion enclosing the anode and having an integral tubular portion at its other end terminating adjacent the open end of the emitter, a baffle structure carried by the shield for providing a circuitous path for discharge from the emitter to the anode and comprising an annular member spanning the anode-cathode space and a disclike member carried by the annular member on the cathode side thereof in spaced overlying relation to the central opening therein, said opening and the space around the periphery of the disclike member defining aperture means through which electron discharge passes, and a firing electrode located within the shield between the bafile structure and the emitter in electrically isolated relation to the other electrodes and the shield and having an effective portion lying in the electron path adjacent the aperture means in the baffie structure where ionization created by the firing electrode will be transferred in a relatively short time to the anode.

3. A thyratron structure comprising a gas-tight envelope, an anode and a cathode located in spaced relation within the envelope, the cathode comprising a hollow cylindrical emitter extending longitudinally of the structure, the emitter being open on the end facing the anode and being electron emissive interiorly, a unitary tubular shield extending substantially coaxial with the emitter with one end portion enclosing the anode and having an integral tubular portion at its other end terminating adjacent the open end of the emitter, a baffie structure carried by the shield for providing a circuitous path for discharge from the emitter to the anode and comprising an annular member spanning the anode-cathode space and a disclike member carried by the annular member on the cathode side thereof in spaced overlying relation to the central opening therein, said opening and the space around the periphery of the disclike member defining aperture means through which electron discharge passes, and a firing electrode located within the shield between the baffle structure and the emitter in electrically isolated relation to the other electrodes and the shield and comprising an annular member lying in the electron path adjacent the peripheral edge of the disclike member where ionization created by the firing electrode will be transferred in a relatively short time to the anode.

4. A thyratron structure comprising a gas-tight envelope, an anode and a cathode located in spaced relation within the envelope, the cathode comprising a hollow cylindrical emitter extending longitudinally of the structure, the emitter being open on the end facing the anode and being electron emissive interiorly, a unitary tubular shield extending substantially coaxial with the emitter with one end portion enclosing the anode and having an integral tubular portion at its other end terminating adjacent the open end of the emitter, a bafiie structure 5. A thyratron structure comprising a gas-tight envelope, an anode and a cathode located in spaced relation Within the envelope, the cathode comprising a hollow cylindrical emitter extending longitudinally of the structure, the emitter being open on the end facing the anode and being electron emissive interiorly, a unitary tubular shield extending substantially coaxial with the emitter with one end portion enclosing the anode and having an integral tubular portion at its other end terminating adjacent the open end of the emitter, the shield being electrically connected to the cathode by a low impedance path, a baffle structure carried by the shield and embodying means for providing a circuitous path for discharge passing from the emitter to the anode, and a firing electrode located within the shield between the bafile structure and the emitter in electrically isolated relation to the other electrodes and the shield and having an effective portion lying in the electron path adjacent said aperture means in the battle structure at a point where ionization created by the firing electrode will be transferred in a relatively short time to the anode.

6. A thyratron structure comprising a gas-tight envelope, an anode and a cathode located in spaced relation within the envelope, the cathode comprising a hollow cylindrical emitter extending longitudinally of the structure, the emitter being open on the end facing the anode.

and being electron emissive interiorly, a unitary tubular shield extending substantially coaxial with the emitter with one end portion enclosing the anode and having its other end integrally joined with the cathode, a bafile structure carried by the shield and embodying means for providing a circuitous path for discharge passing from the emitter to the anode, and a firing electrode penetrating the shield-cathode structure and having an effective portion located within the shield between the baflle struc-- ture and the emitter in electrically isolated relation to the other electrodes and the shield and lying in the electron path adjacent said aperture means in the bafile structure at a point where ionization created by the firing electrode will be transferred in a relatively short time to the anode.

References Cited in the file of this patent UNiTED STATES PATENTS 2,514,165 Ramsay July 4, 1950 2,518,879 Germeshausen Aug. 15, 1950 2,572,881 Rothstein Oct. 30, 1951 2.678.403 Germeshausen May 11, 1954

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